1. Field of the Invention
The present invention relates to a composition comprising an isotactic polypropylene obtained by metallocene catalysis and an isotactic polypropylene obtained by Ziegler-Natta catalysis which is itself grated. This composition is of use as coextrustion tie.
The coextrusion tie comprises grafted isotactic polypropylene obtained by Ziegler-Natta catalysis, diluted in isotactic polypropylene obtained by metallocene catalysis and optionally diluted in at least one polyolefin (E) or in at least one polymer with an elastomeric nature (D) or in at least one blend of (E) and (D).
This coextrusion tie is of use, for example, in the manufacture of multilayer materials for packaging. Mention may be made of materials comprising a film of polyamide (PA) homo- or copolymer and a film of polypropylene (PP) homo- or copolymer, it being possible for the polypropylene film to be laminated onto the polyamide film or coextruded with the polyamide. The coextrusion tie is positioned between the polypropylene film and the polyamide film for good adhesion of the PA and of the PP. These multilayer materials can be three-layer structures PP/tie/EVOH or five-layer structures PP/tie/EVOH/tie/PP in which EVOH denotes a copolymer of ethylene and of vinyl alcohol or a partially or completely saponified ethylene/vinyl acetate (EVA) copolymer.
2. Description of the Related Art
Polypropylene is described in Kirk-Othmer, Encyclopaedia of Chemical Technology, 4th edition, Vol. 17, pages 784-819, John Wiley & Sons, 1996. Virtually all polypropylene sold is composed essentially of isotactic polypropylene obtained by Ziegler-Natta catalysis possibly containing a small amount of atactic polypropylene.
U.S. Pat. No. 5,235,149 discloses packagings closed by caps composed of an aluminium sheet, of a tie layer and of a polypropylene layer. The tie layer of the cap is composed of various polymers grafted with acrylic acid or maleic anhydride; the polymers can be chosen from polyethylene, polypropylene, copolymers of ethylene and of vinyl acetate and copolymers of ethylene and of methyl acrylate.
Patent DE 19 535 915 A discloses a grafted polypropylene block copolymer for adhesively bonding polypropylene films to metal sheets.
Patent EP 689 505 discloses structures similar to those disclosed in the above patent but which are used to prepare food packagings.
Patent EP 658 139 discloses structures similar to those disclosed in the preceding patent but the tie is a grafted random polypropylene copolymer comprising from 1 to 10% of comonomer, the Mw/Mn ratio is between 2 and 10 and the MFI (Melt Flow Index) is between 1 and 20 g/10 min (at 230° C. under 2.16 kg).